HomeMy WebLinkAboutSubsoil Study for Foundation Design 01.11.2021lGrt[iffil[#:1"f#f*ii,"i'*"
An Employce Orncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit Cowrty, Colorado
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SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT F-150 MOUNTAIN MEADOWS
ASPEN GLEN
GARFTELD COUNTY, COLORADO
PROJECT NO.20-7-727
JANUARY ll,202l
PREPARED FOR:
TRIAD PARTNERS,INC.
ATTN: NICK WEAVER
1OO1 SOPRIS MOUNTAIN RANCH ROAD
BASALT, COLORADO 81621
nweaver@triadpartners.net
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TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY.........
PROPOSED CONSTRUCTION
SITE CONDITIONS
SUBSIDENCE POTENTIAL.......
FII,LD EXPLORATION ........
SUBSURFACE CONDITIONS
DESIGN RECOMMENDATIONS ................
FOUNDATIONS
FOUNDATION AND RETAINING WALLS
FLOOR SLABS
UNDERDRAIN SYSTEM
SURFACE DRAINAGE...
LIMITATIONS........._......
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 . GRADATION TEST RESULTS
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Kumar& Associates, lnc. @ Project No.20-7-727
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot F-15, Mountain Meadowso Aspen Glen, Garfield County, Colorado. The project site is
shown on Figure 1. The purpose of the study was to develop recommendations for the
foundation design. The study was conducted in accordance with our agreement for geotechnical
engineering services to Triad Partners, Inc. dated November 24,2020.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils obtained during the field
exploration were tested in the laboratory to determine their classification and other engineering
characteristics. The results of the field exploration and laboratory testing were analyzed to
develop recommendations for foundation types, depths and allowable pressures for the proposed
building foundation. This report summarizes the data obtained during this study and presents our
conclusions, design recommendations and other geotechnical engineering considerations based
on the proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
Building plans for the residence were not available at the time of our study. We assume the
proposed residence to be a one or two-story structure over crawlspace or basement with attached
garage. Ground floors are assumed to be a combination of slab-on-grade and structural over
crawlspace. Grading for the structure is assumed to be relatively minor with cut depths between
about 2 to 4 feet. We assume relatively light foundation loadings, typical of the proposed type of
construction.
If building loadings, location or grading plans change significantly from those described above,
we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The subject site was vacant at the time of our field exploration. The ground surface is sloping
gently down to the south at a grade of about 2 percent. An artificial pond is located north of the
lot on the golf course. Vegetation consists of grass and weeds.
Kumar & Associates, lnc. @ Project No. 20-7-727
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SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen
dcvclopmcnt. Thcsc rocks arc a scqucncc ofgypsifcrous shalc, finc-graincd sandstonc and
siltstone with some massive beds of gypsum and limestone. There is a possibility that massive
gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot.
Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can
produce areas of localized subsidence. During previous work in the areao several sinkholes were
observed scattered throughout the Aspen Glen development. These sinkholes appear similar to
others associated with the Eagle Valley Evaporite in areas of the Roaring Fork River Valley.
Sinkholes were not observed in the immediate area of the subject lot. The nearest mapped
sinkholes are about 300 feet north and 450 feet east of the lot. This lot is located outside the
broad subsidence area mapped about 200 feet east of the lot. No evidence of cavities was
encountered in the subsurface materials; however, the exploratory borings were relatively
shallow, for foundation design only. Based on our present knowledge of the subsurface
conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of
future ground subsidence on Lot F-15 throughout the service life ofthe proposed residence, in
our opinion, is low; however, the owner should be made aware of the potential for sinkhole
development. If further investigation of possible cavities in the bedrock below the site is desired,
we should be contacted.
FIELD EXPLORATION
The field exploration for the project was conducted on November 30,2020. Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions.
The borings were advanced with 4 inch diameter continuous flight augers powered by a truck-
mounted CME-45B drill rig. The borings were logged by a representative of Kumar &
Associates, Inc.
Samples of the subsoils were taken with a I%-inch I.D. spoon sampler. The sampler was driven
into the subsoils at various depths with blows from a 140 pound hammer falling 30 inches. This
test is similar to the standard penetration test described by ASTM Method D-l586. The
penetration resistance values are an indication of the relative density or consistency of the
Kumar & Associates, lnc. @ Project No. 20-7.727
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subsoils. Depths at which the samples were taken and the penetration resistance values are
shown on the Logs of Exploratory Borings, Figure 2. The samples were returned to our
laboratory for review by the project engineer and testing.
SUBSURF'ACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils consist of about 4 to 6 inches of topsoil overlying relatively dense, silty sandy gravel
with cobbles and probably boulders to the maximum explored depth of 1l feet. Drilling in the
dense granular soils with auger equipment was difficult due to the cobbles and boulders and
drilling refusal was encountered in Boring I at 7 feet deep.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and gradation analyses. Results of gradation analyses performed on small diameter drive
samples (minus l%-inch fraction) of the coarse granular subsoils are shown on Figure 3.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist to moist.
DESIGN RECOMMENDATIONS
FOLINDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the building be founded with spread footings bearing
on the natural granular soils.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural granular soils should be designed for
an allowable bearing pressure of 3,000 psf. Based on experience, we expect
settlement of footings designed and constructed as discussed in this section will
be about 1 inch or less.
2) The footings should have a minimum width of l6 inches for continuous walls and
2 feet for isolated pads.
Kumar & Associates, lnc. @ Project No. 20-7-727
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4)
Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations at least 36 inches below exterior grade is typically used in this
atea.
Continuous foundation walls should be reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least 10 feet.
Foundation walls acting as rctaining structures should also be designed to resist
lateral earth pressures as discussed in the "Foundation and Retaining Walls"
section ofthis report.
Topsoil and any loose disturbed soils should be removed and the footing bearing
level extended down to the relatively dense natural granular soils. The exposed
soils in footing area should then be moistened and compacted.
A representative ofthe geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures which are laterally supported and can be expected to
undergo only a slight amount of deflection should be designed for a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of at least 45 pcf for backfill consisting
of the on-site granular soils. Cantilevered retaining structures which are separate from the
residence and can be expected to deflect sufficiently to mobilize the full active earth pressure
condition should be designed for a lateral earth pressure computed on the basis of an equivalent
fluid unit weight of at least 35 pcf for backfill consisting of the on-site granular soils.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The
prcssurcs rccommcndcd abovc assumc draincd conditions bchind thc wolls and a horizontol
backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain
should be provided to prevent hydrostatic pressure buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90% uf the maximurn
standard Proctor density at a moisture content near optimum. Backfill placed in pavement and
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Kumar & Associates, lnc. @ Project No. 20-7-727
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walkway areas should be compacted to at least 95o/o of the maximum standard Proctor density.
Care should be taken not to overcompact the backfill or use large equipment near the wall, since
this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backfill should be expected, even if the material is placed correctly, and could result in distress to
facilities constructed on the backfill. Backfill should not contain organicso debris or rock larger
than about 6 inches.
The lateral resistance of foundation or retaining wall footings will be a combination of the
sliding resistance of the footing on the foundation materials and passive earth pressure against
the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated
based on a coefficient of friction of 0.50. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 375 pcf. The
coefficient of friction and passive pressure values recommended above assume ultimate soil
strength. Suitable factors of safety should be included in the design to limit the strain which will
occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against
the sides of the footings to resist lateral loads should be a granular material compacted to at least
95Yo of the maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction. To reduce the effects of some differential movemento floor slabs should be
separated from all bearing walls and columns with expansion joints which allow unrestrained
vertical movement. Floor slab control joints should be used to reduce damage due to shrinkage
cracking. The requirements for joint spacing and slab reinforcement should be established by the
designer based on experience and the intended slab use. A minimum 4 inch layer of free-
draining gravel should be placed beneath basement level slabs to facilitate drainage. This
material should consist of minus 2-inch aggregate with at least 50% retained on the No. 4 sieve
and less than2%o passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least95%o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site granular soils devoid of vegetation, topsoil and oversized rock.
Kumar & Associates, lnc. @ Project No. 20-7-727
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LINDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
the area that local perched groundwater can develop during times of heavy precipitation or
seasonal runoff. Frozen ground during spring runoff can create a perched condition. We
recommend below-grade construction, such as basement retaining walls and crawlspace areas, be
protected from wetting and hydrostatic pressure buildup by an underdrain system.
The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above
the invert level with free-draining granular material. The drain should be placed at each level of
excavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum l%oto
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
contain less than 2% passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have a
maximum sizeof2 inches. Thedraingravelbackfillshouldbeatleast lYzfeetdeep.
SIJRFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the residence has been completed:
1) Inundation ofthe foundation excavations and underslab areas should be avoided
during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95Yo of the maximum standard Proctor density in pavement and slab areas
and to at least 90% of the maximum standard Proctor density in landscape areas.
3) The ground surface surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. We recommend a minimum
slope of 6 inches in the first 10 feet in unpaved areas and a minimum slope of
2Y"inches in the first 10 feet in paved areas. Free-draining wall backfill should be
covered with filter fabric and capped with about 2 feet of the on-site, finer graded,
soils to rcducc surfacc watcr infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
Kumar & Associates, lnc. @ Project No. 20-7-727
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LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area at this time. We make no warranty either express or implied.
The conclusions and recommendations submitted in this report are based upon the data obtained
from the exploratory borings drilled at the locations indicated on Figure 1, the proposed type of
construction and our experience in the area. Our services do not include determining the
presence, prevention or possibility of mold or other biological contaminants (MOBC) developing
in the future. If the client is concemed about MOBC, then a professional in this special field of
practice should be consulted. Our findings include interpolation and extrapolation of the
subsurface conditions identified at the exploratory borings and variations in the subsurface
conditions may not become evident until excavation is performed. If conditions encountered
during construction appear different from those described in this report, we should be notified so
that re-evaluation of the recommendations may be made.
This report has been prepared for the exclusive use by our client for design pulposes. We are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to verifu that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. We recommend on-site observation
of excavations and foundation bearing shata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfully Submitted,
Kumar & Associates, Inc.lrh
James H. Parsons, E.I.
Reviewed by:
Daniel E. Hardin, P
JHP/kac
Kumar & Associates, lnc, @ Project No. 2A-7-727
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20-7 -727 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
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TOPSOIL: CLAY, SANDY, SILTY, SCATTERED ORcANICS, SOFT, SLIcHTLY MOIST, RED
sAND AND GRAVEL (SU-CU): STLTY, DENSE, SL|GHTLY MO|ST, RED.
i DR|VE SAMPLE, 1 3/8-|NCH r.D. SpLrT SPOON STANDARD PENETRATTON TEST
38/12 7
RIVE SAMPLE BLOW COUNT. INDICATES THAT 38 BLOWS OF A 14o-POUND HAMMER
ALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
t PRACTICAL AUGER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON NOVEMBER 30, 2O2O WITH A 4-INCH-DIAMETER
CONTINUOUS-FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY TAPING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATE
ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6913);
-2oO= PERCENTAGE PASSING NO. 200 SIEVE (ASTM Dl140).
20-7 -727 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
SIEVE ANALYSIS
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MILLIMETERS
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CLAY TO SILT COBBLES
GRAVEL 14 % SAND
LIOUID LIMIT
SAMPLE 0F: Silly Sond ond Grovel
13%
PLASTICITY INDEX
SILT AND CLAY 13 %
FROM:BoringlO4'
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HYDROMETER ANALYSIS
U.S. STANOARO SERIES
,+o 130 118
CLAR SQUARS OPENINGS
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FINE COARSEFINEMEDTUM ICOARSE
20-7 -727 Kumar & Associates GRADATION TEST RESULTS Fig. 3